Field of the Disclosure
The present disclosure relates to an organic light emitting diode display device, and more particularly, to an organic light emitting diode display device and a method of fabricating the same. The organic light emitting diode display device provides uniform brightness and has a large size and high definition.
Description of the Related Art
Recently, flat panel displays have been widely developed and applied to various fields because of their thin profile, light weight, and low power consumption. Among the flat panel displays, organic light emitting diode (OLED) display devices, which can be referred to as organic electroluminescent display devices, emit light during loss of electron-hole pairs. The electron-hole pairs are formed by injecting charges into a light emitting layer between a cathode for injecting electrons and an anode for injecting holes.
The OLED display device can be self-luminous and include a flexible substrate such as plastic. The self-luminous OLED display device can have an excellent contrast ratio and a response time of several micro seconds. The self-luminous OLED display device has advantages in displaying moving images including a displaying the moving images at a wide viewing angle and being stable under low temperatures. The self-luminous OLED display device can be driven by a low voltage of direct current (DC) 5V to 15V, and, as a result, driving circuits in the OLED display can be easily designed and manufactured. In addition, manufacturing processes of the OLED display device can be simple because only deposition and encapsulation steps are required.
In addition, OLED display devices according to driving methods can be passive matrix type OLED display devices and active matrix type OLED display devices. Active matrix type display devices have low power consumption and high definition. In addition, the size of active matrix type display devices can be large.
FIG. 1 is a circuit diagram of one pixel region of an OLED display device according to the related art. The OLED display device includes a gate line GL, a data line DL, a switching thin film transistor Ts, a driving thin film transistor Td, a storage capacitor Cst and a light emitting diode De. The gate line GL and the data line DL cross each other to define a pixel region P. The switching thin film transistor Ts, the driving thin film transistor Td, the storage capacitor Cst and the light emitting diode De are formed in the pixel region P.
More particularly, a gate electrode of the switching thin film transistor Ts is connected to the gate line GL and a source electrode of the switching thin film transistor Ts is connected to the data line DL. A gate electrode of the driving thin film transistor Td is connected to a drain electrode of the switching thin film transistor Ts, and a source electrode of the driving thin film transistor Td is connected to a high voltage supply VDD. An anode of the light emitting diode De is connected to a drain electrode of the driving thin film transistor Td, and a cathode of the light emitting diode De is connected to a low voltage supply VSS. The storage capacitor Cst is connected to the gate electrode and the drain electrode of the driving thin film transistor Td.
The OLED display device can be operated to turn on switching thin film transistor Ts by a gate signal applied through the gate line GL. The switching thin film transistor Ts can be turned on to apply a data signal from the data line DL to the gate electrode of the driving thin film transistor Td and an electrode of the storage capacitor Cst through the switching thin film transistor Ts. When the driving thin film transistor Td is turned on by the data signal, an electric current flowing through the light emitting diode De is controlled, thereby displaying an image. The light emitting diode De emits light due to the current supplied through the driving thin film transistor Td from the high voltage supply VDD.
Namely, the amount of the current flowing through the light emitting diode De is proportional to the magnitude of the data signal, and the intensity of light emitted by the light emitting diode De is proportional to the amount of the current flowing through the light emitting diode De. Thus, the pixel regions P show different gray levels depending on the magnitude of the data signal, and as a result, the OLED display device displays an image.
The storage capacitor Cst maintains charges corresponding to the data signal for a frame when the switching thin film transistor Ts is turned off. Accordingly, even if the switching thin film transistor Ts is turned off, the storage capacitor Cst allows the amount of the current flowing through the light emitting diode De to be constant and the gray level shown by the light emitting diode De to be maintained until a next frame.
OLED display devices include bottom emission type OLED display devices and top emission type OLED display devices depending on an emission direction. In the bottom emission type OLED display devices, light emitted from the light emitting diode is output toward a substrate, where the thin film transistors are formed, through the anode. In the top emission type OLED display devices, light emitted from the light emitting diode is output toward a direction opposite to the substrate through the cathode.
In the OLED display devices, the thin film transistors, generally, are formed under the light emitting diode, and in the bottom emission type OLED display device, an effective emission area is limited by the thin film transistors. The top emission type OLED display device has a larger effective emission area than the bottom emission type OLED display device. Therefore, the top emission type OLED display device has a relatively high aperture ratio as compared with the bottom emission type OLED display device. In addition, the top emission type OLED display device can have a large size and high definition.
In addition, the cathode can be formed of a metallic material. In the top emission type OLED display device, the cathode needs to have a relatively thin thickness to output light through the cathode. As a result, a resistance of the cathode increases, and a VSS voltage drop occurs, thereby causing non-uniform brightness.